Gosh, I would be really frustrated with what has been/is being said to you.

What I would do is swap your simple carbs for more low GI/dense carbs like wholegrain bread instead of white, wholegrain pasta instead of white, avoid rice, eat more vegetables and increase protein, so you feel full / not hungry.

What are you actually eating? I can tell you what I ate in a day b/c I stuck with the same foods most of the time b/c I was avoiding going on insulin.

Breakfast was 2 weet-bix with skim milk and an apple
Mid morning snack with a skim milk cappuccino and a vaalia yoghurt
Lunch - wholegrain sandwich with low fat cheese, avocado & tomato & a banana
Arvo snack - low fat cheese and water crackers
Dinner - Some sort of protein and veg or cup of pasta with bolognaise sauce & salad or veg on the side.
Didn't have dessert
Drank alot of water

Don't worry about the weight gain - I gained hardly anything at all once I was on my GD diet plan & my OBS had no issue with it, in fact he didn't even weigh me, it was only the GD clinic that weighed me. I was also told to eat more in the arvo but I really didn't need it and b/c my levels were fine they stopped checking my diet.

In relation to insulin & the effect it has on the body and eating and insulin etc... I really am clueless. Hopefully someone can come in & explain it to you.